Process for regulating the heating of a thermoplastic band used in a thermoforming station

ABSTRACT

A process for regulating the heating of a thermoplastic band used in a station for thermoforming upwardly open containers, the process being characterized in that, in a zone of the softened thermoplastic band, zone surrounded by a mesh of rigidified matter, the degree of deformation of this zone under the action of a predetermined mass is measured, the measured value of the degree of deformation is compared with a predetermined reference value corresponding to a good aptitude to thermoforming, and the heating of the band is regulated as a function of the value of deformation measured so that the deformation corresponds to the reference value.

This is a division of co-pending application Ser. No. 078,989, filed onJuly 29, 1987, now U.S. Pat. No. 4,758,145, issued 7/19/88.

FIELD OF THE INVENTION

The present invention relates to a process and device for regulating theheating of a thermoplastic band used in a thermoforming station.

BACKGROUND OF THE INVENTION

This invention relates to a process for regulating the heating of athermoplastic band used in a station for thermoforming upwardly opencontainers, of the type in which the thermoplastic band is firstlyheated section by section over the major part of its width except forthin marginal zones adapted to be gripped by a step-by-step advancedevice, up to a softening temperature rendering it suitable forthermoforming, then said band is cooled so as to create therein a sortof meshed network of which the meshes are constituted by rigidifiedthermoplastic matter and each surround a zone of thermoplastic mattersoftened to the thermoforming temperature and corresponding to theextent and location of the upper opening of one of the forming chambersof a molding unit provided in the thermoforming station, and finally,the band thus cooled is advanced by one step into said thermoformingstation, causing the softened zones to coincide with the openings of theforming chambers before proceeding with the thermoforming operationproper.

It has been proven that the softened zones of the thermoplastic band arenot always at a temperature appropriate for thermoforming, with theresult that the containers are torn during thermoforming or present wallzones of considerably unequal thicknesses.

It has already been proposed to measure the temperature of thethermoplastic band when it leaves the heating zone, but the temperaturehas proved not to be representative of the aptitude of the band forthermoforming.

It is an object of the present invention to overcome the drawbacks setforth hereinabove and to propose a process for regulating the heating ofthe thermoplastic band so that it is in all circumstances in the optimumstate of thermoforming.

SUMMARY OF THE INVENTION

In a process of the type mentioned hereinbefore, this object is attainedin accordance with the invention by measuring, in a zone of the softenedthermoplastic band, zone surrounded by a mesh of rigidified matter, thedegree of deformation of this zone under the action of a predeterminedmass, comparing the measured value of the degree of deformation with apredetermined reference value corresponding to a good aptitude tothermoforming, and regulating the heating of the band as a function ofthe value of deformation measured so that said deformation correspondsto the reference value.

Thanks to this design, it is possible to give the thermoplastic band aconstant aptitude to thermoforming by regulating the heating of the bandvirtually continuously, the aptitude to thermoforming being variable asa function of the conditions of thermoforming (ambient temperature,thickness of the band, matter constituting the band, power of absorptionof the heat radiation by the thermoplastic band, variation of thevoltage of the electricity supply network, etc. . . ).

The present invention also relates to a device for regulating theheating of a thermoplastic band used in a station for thermoformingcontainers from a thermoplastic band, of the type comprising a unit foradvancing a thermoplastic band step by step, and, mounted in series, aheating box comprising a plurality of heating resistors, connected to asource of current via a regulating unit, a unit for locally andselectively cooling the band as well as a thermoforming stationcomprising in a molding unit a plurality of upwardly open formingchambers, the cooling unit comprising two alveolate cooling plates ofwhich one is disposed above and the other below the path of advance ofthe band, with the result that the cells of each plate coincide with oneanother and with the zones of band intended for thermoforming andpositioned after an advance step has been made above the openings of theforming chambers.

This type of known device makes it possible to regulate only thetemperature of heating of the thermoplastic band for a predetermineddwell time thereof in the heating box. In this case, it is absolutelyimpossible to take into account, rapidly and virtually instantaneously,the variations of certain parameters which influence the aptitude of theband to thermoforming, such as ambient temperature, thickness of theband, capacity of absorption of the infrared radiation by the band, thechemical composition of the band, etc...

It is therefore a further object of the present invention to propose adevice for regulating the heating of the thermoplastic band to give thelatter a good constant aptitude to thermoforming despite the variationof certain parameters having some influence on the aptitude tothermoforming of the band in question.

This object is attained according to the invention in that the devicefurther comprises a feeler disposed above a cell of the upper alveolateplate and capable of abutting in a cell against the zone ofthermoplastic band delimited by said cell, a displacement sensor capableof measuring the displacement of the feeler whilst the latter stamps ordeforms under the effect of its weight the zone of band in question, acomparator of which one of the inputs is connected to the output of thedisplacement sensor, a generator of a reference signal corresponding toa predetermined length of displacement of the feeler, this generatorbeing connected to the other input of the comparator, and an electronicheating regulator controlled from the outside which is interposed in thesupply circuit of at least a part of the elctrical resistors of theheating box and of which the gate or outer control element is connectedto the output of the comparator.

Thanks to this arrangement, the thermoplastic band is permanentlymaintained at or taken to a heating temperature which gives it aconstant good aptitude to thermoforming, i.e. to stamping, despite thevariations of certain outside parameters. In fact, up to the presenttime, the heating temperature was pre-regulated before leaving thedevice manufacturing workshop at a predetermined value for thermoplasticband having determined physical characteristics for this band to arrivein the thermoforming station with a good aptitude to thermoforming. Thisdevice had to be regulated again as soon as its conditions for use inthe plant manufacturing the containers were different from those of thepreregulation workshop. Similar difficulties occurred when the qualityof the thermoplastic band was changed, etc... All these drawbacks areovercome thanks to the device according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more readily understood on reading the followingdescription with reference to the accompanying drawings, in which:

FIG. 1 is a schematic side view in elevation of an embodiment of thedevice according to the invention.

FIG. 2 is a schematic view of the circuit controlling the supply circuitof the resistors of the heating box.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the drawings, the device for regulating the heating ofa thermoplastic band 1 unwound from a storage reel 2 and advanced stepby step in the direction of arrow 3, comprises a heating box 4presenting inside an enclosure 5 a plurality of heating resistors 6,preferably of the rapid response type. These heating resistors 6 arepreferably disposed longitudinally, in the direction of displacement 3of the band 1, thereabove. However, it is also possible to provideheating resistors below the thermoplastic band 1. The device alsocomprises, immediately downstream--in the direction of advance 3 of thethermoplastic band 1--of the heating box 4, a unit 7 for locally andselectively cooling the thermoplastic band 1 as well as a thermoformingstation 8.

The local and selective cooling unit 7 comprises an upper horizontalalveolate plate 9 of which the cells 11, 12 coincide vertically witheach other and define on the thermoplastic band 1 closed zones fromwhich will be made, at the following thermoforming station 8, containers13 which, after having been stripped, will still be attached to saidband 1. Of course, these containers 13 will subsequently be filled witha product, covered with a sealed lid and cut-out from the thermoplasticband 1. The two alveolate plates 9, 10 are mobile vertically so as to beable to move slightly away from the band or to be able to grip ittherebetween. When these alveolate plates 9, 10 are in contact with thethermoplastic band 1 previously heated to the thermoforming temperature,the corresponding band parts are cooled so as to form a network ofrigidified meshes, whilst the band zones located opposite the cells 11,12 maintain their thermoforming temperature and are surrounded by therigidified meshes of the meshed network.

The thermoforming station 8 comprises, below the thermoplastic band 1, amolding unit 14 which comprises a plurality of forming chambers 15 andis vertically mobile between an upper thermoforming position (cf.FIG. 1) and a lower stripping position in which the containers 13 whichhave just been formed may be advanced by one step with the thermoplasticband to which they are still attached. Above the thermoplastic band 1,the thermoforming station 1 presents a bell 16 connected to a source ofcompressed air and provided at its base with an alveolate holding-downplate 17 which, during thermoforming, is applied against thethermoplastic band 1 supported at the meshes of the network ofrigidified meshes by the upper face of the molding unit 14. Inside thebell 16 are provided a plurality of forming punches 18 of which each isdisposed opposite one of the forming chambers 15 and is fixed, at itsupper end, to a support plate 19 borne by a control rod 20 hermeticallypassing through the upper part of the bell 16 and connected to a controljack (not shown) imparting thereto a reciprocating vertical movementduring the thermoforming operation.

As may be seen in the drawings, the length of an advance step ofthermoplastic band 1 corresponds to the length of the cooling plates 9,10 and to that of the molding unit 14 and holding-down clamp 17, lengthtaken in the direction of advance 3 of the thermoplastic band 1, thelength of the heating box 4 being able to be a whole multiple of thisadvance step.

The device also comprises, above one of the cells 11 of the uppercooling plate 9, a feeler 21 presenting a vertical rod 22 and, ifnecessary, at the upper end thereof, a gripping disc 23, the rod 22being guided vertically in a guiding housing 24 resting, by its lowerend, on the upper cooling plate 9, so that the feeler 21 is centred withrespect to the corresponding cell 11 of said plate 9. In addition, thedevice is provided with a displacement sensor 25 which is fixed in theguiding casing 24, so as to be able to detect the descending movement ofthe rod 22 of the feeler 21, i.e. the degree of deformation or ofstamping of the zone of thermoplastic band 26 delimited by thecorresponding cell of the cooling plate 9, deformation due to the weightof the feeler 21. The structure of the sensor 25 is known per se and maybe of the capacitive, inductive, solenoid plunger, etc... type.

The gripping disc 23 of the feeler 21 is topped by a connecting stirrupelement 27 of which the lower drive fingers 28 are folded radiallytowards the rod 22 of the feeler 21 so as to be able to grip from belowthe gripping disc 23 of the feeler 21 and to lift the latter under theaction of a jack 29 of which the control rod 30 is fast with the upperend of the stirrup 27. Before the end of the descending stroke of thejack 29, the radial fingers 28 of the stirrup 27 are separated from thedisc 23 of the feeler 21 since the lower end of the rod 22 thereof comesinto contact with the thermoplastic band 1 in the zone 26 thereof andthe feeler 21 then deforms this zone 26 under the effect of its ownweight during a determined time less than the time gap separating twosuccessive advance steps of the band 1. The deformation of the zone 26of the band 1 brings about a vertical displacement of the feeler 21,this displacement being measured by sensor 25 provided in the guidingcasing 24. It should be noted that, at the end of descending stroke, theconnecting stirrup 27 rests on the guiding casing 24 and the length ofthe feeler 21 is sufficient in order that, for this lower position ofthe stirrup 27, the disc 23 is still sufficiently apart from the fingers28 of the stirrup 27 and still allows the feeler 21 to descend (cf. FIG.1). Of course, the feeler 21 is raised before each advance of thethermoplastic band 1 and the cooling plate 10 is lowered sufficiently toallow the deformed zone 26 of the said band 1 to pass, whilst, at theend of advance of the band 1, the feeler 21 is again lowered so as torest freely on a zone of band surrounded by a cell 11.

It will be readily understood that, to raise the feeler 21 and to giveit a certain freedom in axial displacement during its penetration intothe thermoplastic band 1, with respect to its lifting means 29, otherconnecting means may be used, for example of the type comprising a jackrod provided with an oblong hole in which is engaged a lateral lug fastwith the upper end of the rod 22 of the feeler 21.

FIG. 2 schematically shows the circuit controlling the descendingdisplacement of feeler 21. This circuit comprises a sensor 25 measuringthe displacement of feeler 21 under its own weight and against theresistance opposed by zone 26 of the thermoplastic band. The output ofsensor 25 is connected to one of the input terminals of a comparator 31of which the other input terminal is connected to the output of areference signal generator 32 for example constituted by a calibrationpotentiometer which furnishes an output value corresponding to a goodaptitude of band 1 to thermoforming.

The output of the comparator 31 is connected to the outside controlmember 33 of an electronic heating regulator such as a static switch 34with outside control which is interposed in the supply circuit 35 of atleast a part of the heating resistors 6 of the heating box 4. Thecomparator 31 is designed for example so that it furnishes at its outputa control signal as long as the signal corresponding to the displacementof the feeler 21 has not attained the value of the reference signalfurnished by the generator 32. In that case, the switch 34 is of thetype which remains closed and establishes the circulation of a supplycurrent as long as its control member 33 receives the output signal ofthe comparator 31. The static switch 34 may be constituted for exampleby a triac, a thyristor or an equivalent electronic circuit. Instead ofemploying a static switch and functioning by all or nothing, anelectronic voltage variator may also be used, controlled by a signal ofwhich the value corresponds to the depth of penetration of the feeler soas to effect regulation of the heating of the band 1 as a function ofthe penetration of the feeler (regulation of the proportional, integral,derivative type).

In order to avoid the heating of the thermoplastic band 1 beingdifferent in the lateral zone and in the central zone, it isadvantageous to provide two different heating circuits, each havingheating resistors 6, a supply circuit 35 and a control circuit 25, 31,32, 33, 34, independent of those of the other heating circuit. In thatcase, a feeler 21 and the sensor 25 corresponding to the first heatingcircuit are disposed opposite a cell 11 in the plate 9, cell locatedopposite the central or median zone of the thermoplastic band 1 andanother feeler 21 and its sensor 25 corresponding to the second heatingcircuit are disposed opposite a cell 11 in the plate 9, cell locatedabove one of the two lateral zones of said thermoplastic band 1, theheating resistors 6 of the first circuit being placed in the heating box4 opposite the central zone of the thermoplastic band 1 and theresistors 6 of the other heating circuit being disposed in two groups ofwhich each is associated with one of the two lateral zones existing oneither side of the central zone of said band 1.

The embodiments described hereinabove may, of course, undergo a certainnumber of modifications without departing from the scope of protectionas defined by the accompanying Claims.

What is claimed is:
 1. A process for regulating the heating of athermoplastic band used in a station for thermoforming upwardly opencontainers, in which the thermoplastic band is first heated section bysection over a major part of its width except for thin marginal zonesadapted to be gripped by a step-by-step advance device, up to asoftening temperature rendering said band suitable for thermoforming,then said band is cooled to create therein a meshed network of which themeshes are constituted by rigidified thermoplastic matter and each meshsurrounds a zone of thermoplastic matter softened to the thermoformingtemperature and corresponding to the extent and location of an upperopening of one of a plurality of forming chambers of a molding unitprovided in the thermoforming station, and finally, the band thus cooledis advanced by one step into said thermoforming station, causing thesoftened zones to coincide with the openings of the forming chambersbefore proceeding with the thermoforming operation,the processcomprising the following steps of:abutting a predetermined mass againstthe zone of the softened thermoplastic measuring, in the zone of thesoftened thermoplastic band, the zone surrounded by the mesh ofrigidified thermoplastic matter, a degree of deformation of this zoneunder the action of the predetermined mass, comparing the measured valueof the degree of deformation with a predetermined reference valuecorresponding to a good aptitude to thermoforming, and regulating theheating of the band as a function of the value of deformation measuredso that said deformation corresponds to the reference value.
 2. Theprocess of claim 1, wherein the zone of which the degree of deformationof the thermoplastic band is measured is a central band zone and theheating of said central zone is controlled as a function of the degreeof deformation measured.
 3. The process of claim 1, wherein the zone ofwhich the degree of deformation of the thermoplastic band is measured isa lateral band zone and the heating of said lateral zone is controlledas a function of the degree of deformation measured.